Hi,

Christian Völker wrote on 2011-07-12 00:17:57 +0200 [Re: [BackupPC-users] My 
Solution for "Off-Site"]:
> On 11/07/2011 21:58, Les Mikesell wrote:
> > The way my 'take a disk off RAID1' works is that there are 3 spare 
> > disks, with at least one always offsite in the rotation [...]
>
> I'm aware of the rotation there- it's just the same and only a question
> on levels you do it. You have three disks and swap them at some time. I
> take snapshots instead. In both cases it can happen a filesystem error
> gets copied over, too.

so, you're saying that you don't trust your file system, but you trust LVM to
keep 4 snapshots accurate for up to four weeks? I think I understand Les'
point (if he's making it) that a hardware-based "don't do anything" approach
is more reliable than a software-based "accumulate the information needed to
undo all my changes". But I also understand your point of "as long as it
works, it gives me three previous states to go back to".

> I think I might move it to the garage, though :)

I hope your data is well enough protected against theft in your garage.

> > [...] to understand how you can drbd to the live partition while keeping 
> > snapshots of old copies.  I wouldn't have expected that to work.  Are 
> > they really layered correctly so the lvm copy-on-write business works?

Why shouldn't it work? An LVM LV is just a block device. Why should the
snapshotting be in any way dependent on the type of data you have on top?

> Yes, this works absolutely fine.  [...] Taking a snapshot of the LVM volume
> doesn't affect the drbd device at all.

I'm just wondering whether you're unmounting the pool FS before the snapshot,
or if you're relying on it to be in a consistent state by itself. How much
testing have you done?

> The only thing I have to evaluate is to have the proper size of the
> snapshot.

Which, in itself, doesn't sound practical. Effectively, you are estimating
how much new data your backups for a week (or four weeks?) will contain.

I just hope you don't decide to implement a BackupPC fork with deduplication
implemented through LVM snapshots ;-).

Regards,
Holger

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